In a conventional lens-shutter camera, it is common for the optical axis of a phototaking lens system and the optical axis of a viewfinder lens system to be separately positioned at a predetermined distance from each other. In recent years, particularly, cameras with a zoom lens having a high zoom ratio have become more common and cameras of even higher magnification have become available.
Most of the camera lenses used for lens-shutter cameras are basic zoom lenses having a focal length range from about 35 mm to about 100 mm for the 35 mm film format. When the zoom ratio becomes high, the focal length at the maximum telephoto end becomes long. Also, it is common for lens-shutter cameras to have a minimum objective distance of about one (1) meter.
In a viewfinder lens system used for lens-shutter cameras, the field angle of the viewfinder system changes according to the change in the picture angle of a phototaking lens system during zooming. When the zoom ratio of the objective lens system increases, the zoom ratio of the viewfinder system increases. The zoom viewfinder lens systems are classified into a real image type viewfinder lens system and a virtual image type viewfinder lens system. Generally, the real image viewfinder lens system is more suitable for a higher zoom ratio and miniaturization than a virtual image type viewfinder lens system. Consequently, real image type zoom viewfinder lens systems dominate today in cameras having a high zoom ratio.
In a real image type zoom viewfinder lens system, an objective lens has a variable focal length, and a movable lens group of the objective lens moves along the optical axis during zooming. The objective lens system is configured such that it overlaps the image position of the object formed with the view frame position only when an object is in a specific camera state within the basic length such as infinitely far (hereafter referred to as "basic length").
Note that within the zoom range from the minimum magnification state, which is the maximum wide view angle, to the maximum magnification state, which is a maximum narrow view angle, the viewfinder magnification is minimal at the wide-angle end. Nonetheless, if the viewfinder magnification is extremely small, it is difficult for a photographer to visually recognize the object. To resolve this, the real image type zoom viewfinder lens system maintains the level of viewfinder magnification with which a photographer can visually recognize the object even at the wide-angle end.
Generally, as the zoom range of a camera lens increases, the focal length of an object lens at the telephoto end increases positively. Therefore, when an object position changes, the image position of the object formed by an object lens changes largely along the optical axis. This displacement of the object image causes a variation of diopter, in which a photographer is unable to observe the object clearly unless he/she adjusts his/her eyes to the object proactively. That is, in a conventional technology, a viewfinder lens system is hard on a photographer's eyes, wearing out his/her eyes. It is difficult to confirm the object with such a viewfinder lens system.
First, the zooming of a phototaking lens system and of a viewfinder lens system of a camera is described as follows:
Conventionally, in a phototaking lens system, a movable lens group for zooming is rotated by a zoom drive system such as motors along a helicoid or cam formed inside a lens barrel. In this way, the focal length of a phototaking lens system is changed during zooming by shifting a movable lens group in the optical axial direction. Generally, a phototaking lens system performs focusing by shifting a predetermined focus lens group in the optical axial direction according to the object position via a focus drive system which is separate from a zoom drive system.
Also, in a viewfinder lens system, a movable lens group for zooming is rotated by a zoom drive system such as motors along a helicoid or cam formed inside a lens barrel. Zooming is performed by shifting a movable lens group along an optical axial system in the same way. Generally, the zoom drive system for a phototaking lens system and for a viewfinder lens system allows miniaturization and power consumption reduction.
If a manufacturing error is ignored, in a phototaking lens system comprising a zoom lens, the image position for an object positioned at infinite distance state is fundamentally always constant when performing a zooming from the maximum wide-angle state, which is in the shortest focal length state to the maximum telephoto state, which is in the longest focal length state. On the other hand, there is the so-called "step-zoom" system in a variable focal length lens system whose image position to an object is not always constant during zooming but is intermittently constant within a predetermined position state (corresponding to a predetermined focal length state.)
In the camera disclosed in Kokai S61-259210, a phototaking lens system is configured with a step-zoom system, which uses a cam orbit except for a predetermined lens position state for focusing. In other words, using a focus cam as a zoom cam, a predetermined focus lens group is moved in the optical axial direction at a predetermined lens position state by means of a zoom drive system to focus on an object. By omitting a focus drive system for a phototaking lens system of a conventional technology, the focusing mechanism incorporating the principles of the present invention is simplified.
As mentioned before, the same zoom drive system for the phototaking lens system drives the phototaking lens movable lens group and the viewfinder lens system zoom lens group. Therefore, when a phototaking lens system focuses on an object at close range, the movable lens group in the viewfinder lens system is automatically driven. As a result, as the phototaking lens system focuses, the viewfinder lens system is triggered to perform a zooming operation, changing the field range of a viewfinder lens system even when the phototaking lens system does not zoom. This is an unfavorable result.
Japanese Laid-Open Kokai H7-5527 discloses a viewfinder lens system which corrects a variation of diopter caused by the change in an object position, through focusing by shifting the lens group nearest to the object side along an optical axis. However, the viewfinder lens system disclosed in Kokai H7-5527 requires separate drive mechanisms for zooming and focusing, complicating the viewfinder lens system configuration.